As the clock rates and the IC density increase in multichip modules (MCMs), power supply noise becomes a more serious problem. It becomes more desirable to position decoupling capacitors closer to the chips and therefore to more effectively isolate the active devices from the switching transients because of the reduced inductance that comes with closer positioning. Approaches to accomplish this include either incorporating a thin film capacitor structure into the multichip module as an integral capacitor or placing the decoupling capacitors between the chip and multichip module, such as on interposer substrates.
One drawback of using decoupling capacitors disposed on interposer substrates is that the amount of chip assembly is doubled. Another drawback is that the chances of solder bump failure can be doubled. A thin film capacitor which is integrated into the body of the multichip module would not have these drawbacks, and therefore is appealing. However, the manufacturing of integrated thin film capacitors faces several technological hurdles before these capacitors can be reliable and economical.
More specifically, an integral capacitor needs to be compatible with the dielectric and metal layers used in the multichip module, and with the processing steps used to form these layers. Therefore, the processing conditions and temperatures used to fabricate the integrated capacitor must not degrade the dielectric and metal layers, and, conversely, the integrated capacitor must have sufficient thermal stability such that its electrical characteristics are not degraded by the formation of the dielectric and metal layers. For example, the formation of a conventional copper-polyimide multichip module requires that each polyimide layer be cured at a high temperature of 300.degree. C. to 450.degree. C. for 30 minutes to 2 hours, depending upon the thickness and chemical composition of the polyimide layer, which would expose an integrated capacitor structure to several extended periods at high temperature.
Additionally, because the integrated capacitor is formed over a large area, defect density of the thin film integrated capacitor is an important characteristic and must be reduced to increase reliability and manufacturing yields. Furthermore, the capacitor should have as high a capacitance value as possible to be the most effective, which requires the use of a high dielectric constant material and/or the use of a very thin dielectric layer. Unfortunately, the use of thin dielectric layers significantly increases defect densities. The use of an anodization material to form the capacitor's dielectric layer offers significant advantages over other formation techniques. Such materials generally provide the highest dielectric constants available for capacitor materials, and the anodization process heals many (but not all) types of defects and enables the dielectric layer to be formed in a controlled and uniform manner. Unfortunately, the base material used to form the anodized oxide dielectric readily diffuses into the dielectric layer when both layers are subjected to high temperatures of 300.degree. C. and above, which is used in the above-described polyimide curing step. The diffusion of the metal creates conductive paths through the dielectric layer, which in turn significantly increases the leakage current of the capacitor. This problem poses a formidable obstacle to successfully manufacturing integrated capacitors.
As an additional consideration, the series resistance in the electrodes of the integrated capacitor needs to be kept low to avoid voltage drops across the capacitor during transient current spikes, and to improve transient response time. This consideration complicates the resolution of the above more serious issues by limiting the available options, and may introduce yet additional problems. As an example of such an additional problem, many researchers have tried to construct a capacitor with aluminum (Al) electrodes for good conductivity and a tantalum pentoxide (Ta.sub.2 O.sub.5) dielectric layer for a high dielectric constant. However, these researchers found that the aluminum chemically reacted with the tantalum pentoxide when the capacitor was heated to 300.degree. C. and significantly increased the leakage current of the capacitor.
Many multichip modules have addressed these challenges by making careful trade-offs between the performance of the module and the performance of the integrated capacitor. For example, thicker dielectric layers in the integrated capacitor have been used to reduce the degradation effects of temperature on the capacitor. However, thicker dielectrics result in lower capacitance, and therefore lower ability to filter out transient current spikes in the power supply. As another trade-off, curing temperatures for the polyimide layers have been reduced. However, the reduced temperature results in a more lossy polyimide dielectric layer, which degrades the performance of the signal lines in the module.
The present invention seeks a way in which these trade-offs can be minimized or avoided so that the electrical performance of the integrated capacitor and the signal lines of the module can be equally maximized.